As a method of production of steel pipe, the method of using a large number of rolls to gradually bend steel plate to finally form it into a round cross-sectional shape, then welding together the end faces is being widely employed.
This bending process is roughly divided into a first breakdown process and a second fin pass process. In the breakdown process, major bending is performed.
Rolling bending of steel plate is performed by pressing steel plate between a top roll and a bottom roll. The steel plate is fed over the bottom roll which shapes the outer side of the steel plate and is bent by the top roll which shapes the inner side of the steel plate.
As shown in FIG. 1, both the top roll 1 and the bottom roll 2 usually have a roll caliber with a curvature along a bending direction of the steel plate 3.
As shown in FIG. 2, the bottom roll 2 sometimes also has a straight roll caliber.
When roll bending thick-gauge steel plate, in both the case of FIG. 1 and FIG. 2, at the time of starting bending, the steel plate edges 4 contact the bottom roll 2 in a line (or by points). If a downward pressing force is applied, the steel plate edges 4 unavoidably crush.
Once edge corner deformation occurs, restoration is impossible.
When the drawing action in the fin pass process shown in FIG. 3A is insufficient, if the steel plate edges 4 are made to abut in a pipe shape, as shown in FIG. 3B, a Y-shaped joint will be formed resulting in a welding defect.
If sufficient drawing was applied in the fin pass process, as shown in FIG. 3C, the edges will locally increase in thickness and the circularity of the inside circumference of the pipe will sometimes fall.
This problem becomes pronounced when t/D, where the plate thickness of the steel pipe is “t” and the diameter is D, is 0.06 or more.
To avoid this problem, in PLT 1, as shown in FIG. 4A or FIG. 4B, the method is proposed of pre-bending the steel plate edges 4 in advance by pre-bend-use rolls so that in the breakdown process shown in FIG. 4C, the steel plate edges 4 are not bent.
However, even if using pre-bend-use rolls, at the time of start of bending, no matter what is done, a certain extent of edge corner deformation is unavoidable. Further, before the breakdown process, it is then necessary to install a pre-bend-use stand, so the problem of inviting a larger size of the facilities remains.